The long-range goal of this research program is to characterize how much of a grammar is attainable in the context of cognitive limitations that last throughout the lifespan. The research focuses on people with Williams syndrome (WS)--a neurodevelopmental genetic disorder which results in moderate mental retardation, but impressively good language skills. Early evidence suggested that the WS profile could be taken as evidence for language modularity. However, recent research has documented poor performance in some aspects of WS morphosyntax, and these findings have been taken to challenge the modularity claim. Some researchers have even implied that we can expect to learn nothing about how language acquisition normally works by studying WS. But the potential implications of WS for understanding language acquisition do not depend on perfect language performance. Poor performance does not necessarily reflect impairments in underlying knowledge. The crucial question with regard to WS is not whether cognitive limitations have an influence in how one performs in tests of language abilities, but rather, what kind of grammatical competence system is one capable of achieving in the context of cognitive limitations? To answer this question, it is critical to localize the source of any problems as either residing in the competence system or in the performance systems that are called upon in comprehension, production, judgment, etc. The overall aim of the proposed work is to flesh out the linguistic profile of people with WS in both breadth and depth, in order to inform these questions. This aim is achieved by both investigating poor performance in specific language tasks in order to identify the locus of the difficulty, and by examining specific new areas of linguistic knowledge in this population.